The tremendous increase in connectivity of computers has increased the importance of a robust security infrastructure. A computer system's security features can be used to prevent unauthorized access (e.g., from the Internet). A secure execution environment that preserves the privacy and integrity of transactions is important to enable the use of Internet based commerce. A secured execution environment could also be used to provide copy protection for published software or content (e.g., movies) stored on media like Digital Versatile Disks (DVDs) or Compact Disk-Read Only Memories (CD-ROMs).
Securing execution includes ensuring the integrity of the execution and ensuring the privacy of code and data. Various types of threats may lead to a compromise of the integrity or privacy of a system. For example, malicious software may be able to exploit weaknesses in the operating system. Direct memory access devices may be able to read physical memory without processor support. A logic analyzer may be used to observe the traffic between the processor and the memory. Attacks may also be made which take advantage of a processor's built-in debug mode or probe mode or which physically modify the connectivity of components in the system to observe and modify the communication between the components. An attacker could also subject the hardware to an abnormal voltage, temperature or frequency so as to compromise the execution of the system and possibly cause hardware to “leak” out secrets. In addition, an attacker could remove the process layers selectively to expose the device structures hiding the secrets or use an Ion beam to examine the flow of signals inside the device.
Some security schemes, such as those based on public/private key cryptography, uses private data or code that needs to be kept secret. In several cases, the encoding algorithm used is publicly known, and the strength of the cryptography is in keeping the key secret. Intermediate data, such as the control flow information of the program, should not be exposed because it may make it easier to determine the private key. In this regard, conditional assignment capability of a processor architecture may be used to eliminate branches on the secret data, but it is difficult to design code to meet this requirement. For some applications, part of the cryptographic strength is in the privacy of the algorithm itself In this case, the execution environment needs to guarantee that even the execution code, along with the input data and all intermediate results, is kept private.
In some systems, a special security coprocessor is used to protect the code and data of a host processor. Known systems do not provide an effective means for a host processor to protect the private code or data (e.g., a key) within the host processor against attacks that use, for example, malicious software, direct memory access devices, logic analyzers, exploit the system debug/probe mode, or modify the hardware to reveal secrets.